1. Field of the Invention
The present invention relates to a method of fabricating a thermoelectric device for use in a thermoelectric power generator taking advantage of the Seebeck effect, or a cooler taking advantage of the Peltier effect, and more particularly, to a method of fabricating a small sized thermoelectric device incorporating a plurality of thermocouples.
2. Description of the Related Art
In each of the thermocouples making up a thermoelectric device, a voltage is developed by providing a difference in temperature between the opposite ends thereof. This is due to the effect of the Seebeck effect, and a device designed to extract the voltage as electric energy is a thermoelectric power generator. The thermoelectric power generator wherein heat energy can be converted directly into electric energy has attracted much attention as effective means of utilizing heat energy, as represented by the case of waste heat utilization.
Meanwhile, the flow of a current caused to occur through a thermocouple results in generation of heat at one end thereof, and absorption of heat at the other end thereof. This is due to the Peltier effect, and a cooler can be manufactured by taking advantage of such phenomenon of heat absorption. This type of cooler which does not comprise mechanical components and can be reduced in size has an application as a portable refrigerator, or a localized cooler for lasers, integrated circuits, and the like.
Thus, the thermoelectric power generator or cooler made up of the thermoelectric device is simple in construction, and is in a more favorable condition for miniaturization as compared with other types of power generators or coolers, offering high usefulness. For example, with the thermoelectric device for use in the thermoelectric power generator, there will not arise a problem of leakage or depletion of electrolyte as with the case of a redox cell, and the thermoelectric device has therefore promising prospects for application to portable electronic devices such as an electronic wrist watch.
The general construction of a conventional thermoelectric device, and a conventional method of fabricating the same, have been disclosed in, for example, Japanese Patent Laid-open Publication No. 63-20880 or Japanese Patent Laid-open Publication No. 8-43555. The descriptions disclosed therein are concerned with a thermoelectric device for use in generation of power. However, the basic construction thereof is the same as that of a thermoelectric device for use in cooling. Hence, the thermoelectric device for use only in generation of power is described hereinafter to avoid complexity in explanation.
In the conventional thermoelectric device disclosed in the publications described above, p-type and n-type thermoelectric semiconductors are alternately and regularly arranged so that a multitude of thermocouples are formed on a horizontal plane, and the thermocouples thus formed are electrically connected to each other in series.
The thermoelectric device is formed in a sheet-like shape by disposing the thermocouples on a plane, and the upper surface and under surface of the thermoelectric device become faces on which hot junctions and cold junctions of the thermocouples are located, respectively. Generation of electric power in the thermoelectric device is caused to occur by a difference in temperature between the upper surface and the under surface of the device having a sheet-like shape.
Meanwhile, an output voltage of a thermocouple using a BiTe-based material, said to have the highest figure of merit of thermoelectric power generation at present, is about 400 .mu.V/.degree. C. per couple.
When such thermocouples as described above are employed in a portable electronic device for use at around room temperature, for example, in an electronic watch, a satisfactory difference in temperature can not be expected to be developed inside the device. For example, in the case of a wrist watch, the temperature difference in a wrist watch developed between body temperature and the ambient temperature will be 2.degree. C. at most.
It follows that not less than about 2000 couples of BiTe-based thermocouples are required to obtain a voltage not lower than 1.5V, necessary for driving an electronic watch.
Furthermore, in the case of an electronic wrist watch, wherein mechanical components and electric circuit components need to be encased therein in spite of a small internal volume thereof in the first place, it is required that a thermoelectric device for power generation, very small in size, be used.
The conventional method of fabricating a thermoelectric device small in size and composed of a multitude of thermocouples has been disclosed in Japanese Patent Laid-open Publication No. 63-20880.
In the method disclosed, a multi-layered body is formed by stacking p-type and n-type thermoelectric semiconductors, in a thin sheet-like shape, on top of each other in layers while interposing a heat insulating material between respective layers, and then by bonding them together. Subsequently, grooves are formed at a given spacing in the multi-layered body, whereupon the grooves are filled up with a heat insulating material, and connecting portions of individual thermoelectric semiconductors are removed, thereby forming n-type and p-type thermocouples, surrounded by the heat insulating material and isolated from each other. By electrically connecting the thermocouples with each other in series, a thermoelectric device is completed.
Then, in the method disclosed in Japanese Patent Laid-open Publication No. 8-43555, p-type and n-type thermoelectric semiconductors, each having a plate-like shape, are first bonded to separate substrates, and thereafter, a grooving process of forming a multitude of grooves at very small spacings in the longitudinal and transverse directions is applied to respective thermoelectric semiconductors.
As a result of the grooving process described above, a multitude of thermoelectric semiconductors, each columnar in shape, and upstanding regularly on top of the respective substrates, resembling a kenzan (a needle-point flower holder for flower arrangement), are formed. The kenzan-like bodies composed of the n-type and p-type thermoelectric semiconductors, respectively, are thus prepared, and joined together such that the respective thermoelectric semiconductors, columnar in shape, are mated with each other. Thereafter, an insulating material is filled between the respective thermoelectric semiconductors.
In the final step of processing, the substrates are removed, and a thermoelectric device is completed by electrically connecting thermocouples with each other in series.
However, with the methods of fabricating the thermoelectric device as described in the foregoing, there will arise a problem that the material used for the thermoelectric semiconductors is prone to breakage during the process of forming the thermoelectric semiconductors into a sheet-like shape, during the grooving process of forming the kenzan-like bodies, and the like, because of the fragile nature of the material itself for the thermoelectric semiconductors.
In particular, for forming as many as not less than 2000 couples of thermocouples in an ultra-small sized thermoelectric device which can be encased in a wrist watch, it is required that the thickness of the respective sheet-like thermoelectric semiconductors or the diameter of the respective columnar thermoelectric semiconductors be set to on the order of 100 .mu.m or less, and consequently, the problem of fragility described above will become quite serious.